The reliability of radiographic assessment of femoral neck-shaft and implant angulation in hip resurfacing arthroplasty

Hip Resurfacing: A Single Surgeon U.S. Series With Minimum Ten-Year Follow-up

BACKGROUND

Metal-on-metal hip resurfacing is an alternative to total hip arthroplasty (THA). The aim of this study was to determine implant survivorship, analyze patient-reported outcomes measures and to determine patient satisfaction for patients who underwent metal-on-metal hip resurfacing at a large US academic institution by a single surgeon with a minimum of 10-year follow-up.

METHODS

Patients who underwent hip resurfacing from September 2006 through November 2009 were included. Patient demographics and variables were collected from a prospectively maintained institutional database and patients completed an additional questionnaire with patient-reported outcomes measures.

RESULTS

A total of 350 patients (389 hips) out of 371 (433 hips) with a minimum 10-year follow-up were successfully contacted (94.3% follow-up). Mean age was 53 years, 258 were male (73%). 377 out of 389 hips (96.9%) did not require additional surgery. Gender was significantly related to implant survivorship (males 99.0%, females 90.9%; P < .001). 330 patients (369 hips, 94.8%) were satisfied with their surgery. Males had higher proportion of satisfaction scores (P = .02) and higher modified Harris Hip Score (odds ratio = 2.63 (1.39, 4.98), P = .003). Median modified Harris Hip Score score for non-revised hips was 84.0 [80.0; 86.0] versus those requiring revision, 81.5 [74.0; 83.0], (P = .009).

CONCLUSION

At a minimum 10-year follow-up, hip resurfacing, using an implant with a good track record, demonstrates 99.0% survivorship in male patients with an average age of 52 years. We believe that the continued use of metal-on-metal hip resurfacing arthroplasty in this population is justified by both positive patient reported outcomes and survivorship.

Comparing two different automatic methods to measure femoral neck-shaft angle based on PointNet++ network

Accurate measurement of the femoral neck-shaft angle (NSA) is of great significance for diagnosing hip joint diseases and preoperative planning of total hip arthroplasty. However, the repeatability of manual measurements is not as satisfactory, and the difference between 2D and 3D measurements is not clear. The computer-aided method provides a platform for automatic and accurate measurement of the NSA. The femoral point cloud datasets from 310 subjects were segmented into three regions, including the femoral head, femoral neck, and femoral shaft using PointNet++. We created a projection plane to simulate the hip anteroposterior radiograph and fitted the femoral neck axis and femoral shaft axis to complete the 2D measurement, while we directly fitted the two axes in space to complete the 3D measurement. Also, we conducted the manual measurement of the NSA. We verified the accuracy of the segmentation and compared the results of the two automatic and manual methods. The Dice coefficient of femoral segmentation reached 0.9746, and MIoU of that was 0.9165. No significant difference was found between any two of the three methods. While comparing the 2D and 3D methods, the average accuracy was 98.00%, and the average error was 2.58°. This paper proposed two accurate and automatic methods to measure the NSA based on a 2D plane and a 3D model respectively. Although the femoral neck and femoral shaft axes did not intersect in 3D space, the NSAs obtained by 2D and 3D methods were basically consistent.

Femoral Neck Shaft Angle and Management of Proximal Femur Fractures: Is the Contralateral Femur a Reliable Template?

OBJECTIVES

To (1) assess interrater reliability of a novel technique for measurement of neck shaft angle (NSA); (2) use pelvic anteroposterior (AP) radiographs of unaffected hips to assess variability of NSA; and (3) evaluate the side-to-side variability of NSA to determine reliability of using the contralateral hip as a template.

DESIGN

Retrospective cohort study.

SETTING

Academic Level 1 regional trauma center.

PATIENTS/PARTICIPANTS

Four hundred six femora (203 patients) with standing AP pelvis radiographs were selected. Exclusions included lack of acceptable imaging, congenital abnormalities, or prior hip surgery.

INTERVENTION

An AP pelvis radiograph in the standing position.

MAIN OUTCOME MEASUREMENTS

Bilateral NSA measurements obtained in a blinded fashion between 2 reviewers. Pearson coefficients and coefficient of determination assessed correlations and variability between left and right NSA. Concordance correlation coefficients assessed the interrater reliability between measurements performed by the 2 reviewers.

RESULTS

Two hundred three patients (406 femora) were assessed. Male patients had a lower overall NSA mean of 131.56 degrees ± 4.74 than females with 133.61 degrees ± 5.17. There was no significant difference in NSA side-to-side in females (P = 0.18), 0.3 degrees [95% confidence interval (-0.15 to 0.75)], or males (P = 0.68), 0.19 degrees [95% confidence interval (-0.74 to 1.12)]. There was a strong linear relationship between left and right femora (r2 = 0.70). Forty-one percent of patients fell within the 131-135 degrees range bilaterally. Eighty-eight percent of patients had <5 degrees difference in NSA bilaterally and 0% had >10 degrees difference.

CONCLUSIONS

There is no significant variability between bilateral femora in males and females. Use of this measurement method and contralateral NSA for proximal femur fracture planning is supported.

Pre-operative templating in THA. Part II: a CT-based strategy to correct architectural hip deformities

INTRODUCTION

Pre-operative templating for total hip arthroplasty (THA) remains inaccurate due to improper magnification and alignment. We aimed to describe an improved templating strategy using computed tomography (CT) to predict component sizes and offsets with greater accuracy.

MATERIALS AND METHODS

We analysed 184 CT images acquired for pre-operative templating of primary THA. We aimed to restore native (pre-arthritic) femoral offset and limb length, by raising the head center to the level of the templated cup center cranio-caudally, but maintaining the pathologic (pre-operative) head center medio-laterally (except in medialized hips). Acetabular offset (AO) and femoral offset (FO) were measured on pre-operative CT scans, during acetate templating, and on post-operative true antero-posterior radiographs.

RESULTS

The post-operative offsets were within ± 5 mm from templated estimates in 174 hips (91%) for AO, in 116 hips (61%) for FO, in 111 hips (58%) for GO, and in 134 hips (70%) for neck cut level. The post-operative hip architecture reproduced the templated hip architecture within ±5 mm in 77 hips (40%). The agreement between planned and post-operative parameters was moderate for stem size (0.57), cup size (0.62), AO (0.50), but fair for FO (0.45). The AO decreased in most arthritic types, notably in lateralized hips (6.6 mm), but remained unchanged in medialized hips. The FO increased in most arthritic types (1.8-3.1 mm) but remained unchanged in medialized and lateralized hips.

CONCLUSIONS

We described a strategy for pre-operative templating in THA. Despite the accuracy of CT, the authors found significant variations between planned and post-operative reconstructions, which suggest that pre-operative templating should only be used as an approximate guide.

Validation of Neck-Shaft Angle Correction After Cephalomedullary Nail Fixation

OBJECTIVES

To validate a neck-shaft angle (NSA) correction formula using the known angle between the medullary nail and lag screw of an indwelling cephalomedullary nail (CMN).

METHODS

Pertrochanteric femur fractures were created in 7 synthetic femurs and reconstructed with a CMN. Anteroposterior x-rays were taken with the proximal femur in 0, 15, 30, and 45 degrees of internal/external rotation and 10 and 20 degrees of flexion with neutral rotation. The femoral NSA and the nail angle were measured on anteroposterior x-rays. The corrected NSA was obtained by multiplying the known nail angle by the quotient of the measured NSA over the measured nail angle.

RESULTS

With increasing degrees of internal or external rotation, the difference between the actual and the measured NSA increased. After correction using the known angle of the nail, the measured NSA was reduced to within 3 degrees of the actual NSA over all degrees of rotation. At neutral rotation, 15 degrees of external rotation, and 10 and 20 degrees of hip flexion, there was minimal difference between the actual NSA and the measured NSA, negating the effect of the correction formula.

CONCLUSIONS

The measured NSA after CMN of intertrochanteric fractures is altered by hip rotation. This study validates the correction formula, which determines this difference by using the known angle between the lag screw and the nail.

The Reliability of Sensor-Assisted Soft Tissue Measurements in Primary Total Knee Arthroplasty

BACKGROUND

In pursuit to improve soft tissue balancing in total knee arthroplasties (TKAs), a wireless device was introduced to assess femorotibial pressures. The aim of this study was to evaluate the reliability of this device.

METHODS

After 33 TKAs were balanced by conventional techniques, contact pressures were measured using a wireless sensor 3 times in a row; twice while the examiner was blinded for the result (n = 29); and once while the examiner was able to see the result as visual feedback (n = 32). Femorotibial pressures were measured in the medial and lateral compartments with the knee in 10°, 45°, and 90° of flexion (6 measurements per TKA). Furthermore, both the combined pressure and the difference in pressure between the compartments was calculated throughout the 3 positions (together another 6 measurements per TKA).

RESULTS

The intraclass correlation coefficient between the blind measurements was poor in 2 of the 12 (17%), moderate in 4 of 12 (33%), and good in 6 of 12 (50%) measurements. The intraclass correlation coefficient between the blind and observing measurement was poor in 2 of the 12 (17%), moderate in 6 of 12 (50%), and good in 4 of 12 (33%) measurements. Especially measurements in 10° of flexion are associated with poorer reliability.

CONCLUSION

The wireless sensor has a moderate to good reliability in 83% of the measurements.

Validity and reliability of different techniques of neck-shaft angle measurement

AIM

To determine a valid and reliable neck-shaft angle (NSA) measurement method while rotating the pelvises in increments of 5° in order to simulate patient malpositioning.

MATERIALS AND METHODS

CT images of 17 patients were used to produce digitally reconstructed radiographs in frontal and lateral views and three-dimensional (3D)-reconstructions of the femurs, considered to be the reference standard. Malpositioning was simulated by axially rotating the frontal radiographs from 0° to 20°. Three operators measured in two-dimensions the NSA using four different methods, three times each, at each axial rotation (AR) position. Method 1 (femoral neck axis drawn by joining the centre of the femoral head (CFH) to the median of the femoral neck base; femoral diaphysis axis drawn by joining the median of two lines passing through the medial and lateral edges of the femoral axis below the lesser trochanter) and method 2 (femoral axis taken as the median of a triangle passing through base of femoral neck and medial and lateral head-neck junction; femoral diaphysis as previous) were described for the first time; method 3 was based on a previous study; method 4 was a free-hand technique. Reliability, validity, and global uncertainty were assessed.

RESULTS

Method 1 showed the best reliability and validity. The global uncertainty also showed minimal values for method 1, ranging from 7.4° to 14.3° across AR positions.

CONCLUSION

Method 1, based on locating the CFH, was the most reliable and valid method and should be considered as a standardised two-dimensional NSA measurement method for clinical application.

Effect of combined flexion and external rotation on measurements of the proximal femur from anteroposterior pelvic radiographs

INTRODUCTION

Fixed flexion and external rotation contractures are common in patients with hip osteoarthritis and, in particular, before total hip replacement (THR). We aimed to answer the following question: how does combined flexion and external rotation of the femur influence the radiographic assessment of (1) femoral offset (FO) (2) neck-shaft angle (NSA) and (3) distance (parallel to the femoral axis) from greater trochanter to femoral head center (GT-FHC)?

HYPOTHESIS

Combined flexion and external rotation impact the accuracy of two-dimensional (2D) proximal femur measurements.

MATERIALS AND METHODS

Three-dimensional (3D) CT segmentations of the right femur from 30 male and 42 female subjects were acquired and used to build a statistical shape model. A cohort (n=100; M:F=50:50) of shapes was generated using the model. Each 3D femur was subjected to external rotation (0°-50°) followed by flexion (0°-50°) in 10° increments. Simulated radiographs of each femur in these orientations were produced. Measurements of FO, NSA and GT-FHC were automatically taken on the 2D images.

RESULTS

Combined rotations influenced the measurement of FO (p<0.05), NSA (p<0.001), and GT-FHC (p<0.001). Femoral offset was affected predominantly by external rotation (19.8±2.6mm [12.2 to 26.1mm] underestimated at 50°); added flexion in combined rotations only slightly impacted measurement error (20.7±3.1mm [13.2 to 28.8mm] underestimated at 50° combined). Neck-shaft angle was reduced with flexion when external rotation was low (9.5±2.1° [4.4 to 14.2°] underestimated at 0° external and 50° flexion) and increased with flexion when external rotation was high (24.4±3.9° [15.7 to 31.9°] overestimated at 50° external and 50° flexion). Femoral head center was above GT by 17.0±3.4mm [3.9 to 22.1mm] at 50° external and 50° flexion. In contrast, in neutral rotation, FHC was 12.2±3.4mm [3.9 to 22.1mm] below GT.

DISCUSSION

This investigation adds to current understanding of the effect of femoral orientation on preoperative planning measurements through the study of combined rotations (as opposed to single-axis). Planning measurements are shown to be significantly affected by flexion, external rotation, and their interaction.

LEVEL OF EVIDENCE

IV Biomechanical study.

Does the Angle of the Nail Matter for Pertrochanteric Fracture Reduction? Matching Nail Angle and Native Neck-Shaft Angle

OBJECTIVES

To determine whether fixation of pertrochanteric hip fractures with cephalomedullary nails (CMNs) with a neck-shaft angle (NSA) less than the native NSA affects reduction and lag screw cutout.

DESIGN

Retrospective comparative study.

SETTING

Level I trauma center.

PATIENTS/PARTICIPANTS

Patients treated with a CMN for unstable pertrochanteric femur fractures (OTA/AO 31-A2.2 and 31-A2.3) between 2005 and 2014.

INTERVENTION

CMN fixation.

MAIN OUTCOME MEASUREMENT

NSA reduction and lag screw cutout.

RESULTS

Patients fixed with a nail angle less than their native NSA were less likely to have good reductions [17% vs. 60%, 95% confidence interval (CI), -63% to -18%; P = 0.0005], secondary to more varus reductions (41% vs. 10%, 95% CI, 9%-46%; P = 0.01) and more fractures with ≥4 mm of displacement (63% vs. 35%, 95% CI, 3%-49%; P = 0.03). The cutout was not associated with the use of a nail angle less than the native NSA (60% vs. 76%, 95% CI, -56% to 18%; P = 0.5), varus reductions (60% vs. 32%, 95% CI, -13% to 62%; P = 0.3), or poor reductions (20% vs. 17%, 95% CI, -24% to 44%; P = 1.0).

CONCLUSIONS

The fixation of unstable pertrochanteric hip fractures with a nail angle less than the native NSA was associated with more varus reductions and fracture displacement but did not affect the lag screw cutout.

LEVEL OF EVIDENCE

Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

The femoral neck-shaft angle on plain radiographs: a systematic review

OBJECTIVE

The femoral neck-shaft angle (NSA) is an important measure for the assessment of the anatomy of the hip and planning of operations. Despite its common use, there remains disagreement concerning the method of measurement and the correction of hip rotation and femoral version of the projected NSA on conventional radiographs. We addressed the following questions: (1) What are the reported values for NSA in normal adult subjects and in osteoarthritis? (2) Is there a difference between non-corrected and rotation-corrected measurements? (3) Which methods are used for measuring the NSA on plain radiographs? (4) What could be learned from an analysis of the intra- and interobserver reliability?

MATERIAL AND METHODS

A systematic literature search was performed including 26 publications reporting the measurement of the NSA on conventional radiographs.

RESULTS

The mean NSA of healthy adults (5,089 hips) was 128.8° (98-180°) and 131.5° (115-155°) in patients with osteoarthritis (1230 hips). The mean NSA was 128.5° (127-130.5°) for the rotation-corrected and 129.5° (119.6-151°) for the non-corrected measurements.

CONCLUSION

Our data showed a high variance of the reported neck-shaft angles. Notably, we identified the inconsistency of the published methods of measurement as a central issue. The reported effect of rotation-correction cannot be reliably verified.

The impact of proximal femoral morphology on failure strength with a mid-head resection short-stem hip arthroplasty

Mid-head resection short-stem hip arthroplasty is a conservative alternative to conventional total hip replacement and addresses proximal fixation challenges in patients not suitable for hip resurfacing. It is unclear whether proximal femoral morphology impacts the ultimate failure load of mid-head resection implanted femurs, thus the aim of this study was to investigate the effect of native neck-shaft angle (NSA) and coronal implant alignment on proximal femoral strength. In total, 36 synthetic femurs with two different proximal femoral morphologies were utilized in this study. Of them, 18 femurs with a varus NSA of 120° and 18 femurs with a valgus NSA of 135° were each implanted with a mid-head resection prosthesis. Femurs within the two different femoral morphology groups were divided into three equal coronal implant alignment groups: 10° valgus, 10° varus or neutral alignment. Prepared femurs were tested for stiffness and to failure in axial compression. There was no significant difference in stiffness nor failure load between femurs implanted with valgus-, varus- or neutrally aligned implants in femurs with a NSA of 120° (p = 0.396, p = 0.111, respectively). Femurs implanted in valgus orientation were significantly stiffer and failed at significantly higher loads than those implanted in varus alignment in femurs with a NSA of 135° (p = 0.001, p = 0.007, respectively). A mid-head resection short-stem hip arthroplasty seems less sensitive to clinically relevant variations of coronal implant alignment and may be more forgiving upon implantation in some femoral morphologies, however, a relative valgus component alignment is recommended.

Evaluation of a patient specific femoral alignment guide for hip resurfacing

A novel alternative to conventional instrumentation for femoral component insertion in hip resurfacing is a patient specific, computed tomography based femoral alignment guide. A benchside study using cadaveric femora was performed comparing a custom alignment guide to conventional instrumentation and computer navigation. A clinical series of twenty-five hip resurfacings utilizing a custom alignment guide was conducted by three surgeons experienced in hip resurfacing. Using cadaveric femora, the custom guide was comparable to conventional instrumentation with computer navigation proving superior to both. Clinical femoral component alignment accuracy was 3.7° and measured within ± 5° of plan in 20 of 24 cases. Patient specific femoral alignment guides provide a satisfactory level of accuracy and may be a better alternative to conventional instrumentation for initial femoral guidewire placement in hip resurfacing.

Femoral neck-shaft angle in humans: variation relating to climate, clothing, lifestyle, sex, age and side

The femoral neck-shaft angle (NSA) varies among modern humans but measurement problems and sampling limitations have precluded the identification of factors contributing to its variation at the population level. Potential sources of variation include sex, age, side (left or right), regional differences in body shape due to climatic adaptation, and the effects of habitual activity patterns (e.g. mobile and sedentary lifestyles and foraging, agricultural, and urban economies). In this study we addressed these issues, using consistent methods to assemble a global NSA database comprising over 8000 femora representing 100 human groups. Results from the analyses show an average NSA for modern humans of 127° (markedly lower than the accepted value of 135°); there is no sex difference, no age-related change in adults, but possibly a small lateral difference which could be due to right leg dominance. Climatic trends consistent with principles based on Bergmann's rule are evident at the global and continental levels, with the NSA varying in relation to other body shape indices: median NSA, for instance, is higher in warmer regions, notably in the Pacific (130°), whereas lower values (associated with a more stocky body build) are found in regions where ancestral populations were exposed to colder conditions, in Europe (126°) and the Americas (125°). There is a modest trend towards increasing NSA with the economic transitions from forager to agricultural and urban lifestyles and, to a lesser extent, from a mobile to a sedentary existence. However, the main trend associated with these transitions is a progressive narrowing in the range of variation in the NSA, which may be attributable to thermal insulation provided by improved cultural buffering from climate, particularly clothing.

Reproducibility of radiographic assessment of femoral implant position after hip resurfacing arthroplasty: a pilot study

To determine whether femoral implant position can be reproducibly measured on plain digital radiographs, we prospectively studied 40 patients after hip resurfacing arthroplasty. Three observers performed double blinded randomized analysis of calibrated digital radiographs meeting strict quality criteria. The implant stem-shaft angle and femoral anteversion angle were measured by the trapezoid method of axis determination using OsiriX software. The upper and lower offset and the anterior and posterior offset were measured. The statistical analysis was performed using Pearson correlation tests (intra-observer reproducibility) and Fisher F tests (inter-observer reproducibility). Intra-observer reproducibility was very good for all parameters and all observers. Inter-observer reproducibility was excellent except for superior offset measurement. Thus, this study validates a radiographic method for assessing the femoral implant position in hip resurfacing. We believe this could be useful for future studies on hip resurfacing devices.

Do survival rate and serum ion concentrations 10 years after metal-on-metal hip resurfacing provide evidence for continued use?

BACKGROUND

Owing to concerns attributable to problems associated with metal-on-metal bearing surfaces, current evidence for the use of hip resurfacing is unclear. Survival rates reported from registries and individual studies are controversial and the limited long-term studies do not conclusively allow one to judge whether hip resurfacing is still a reasonable alternative to conventional THA.

QUESTIONS/PURPOSES

We asked whether the long-term survival rate of hip resurfacing is comparable to that of conventional THA and certain factors can be identified that influence serum ion concentration 10 years postoperatively. We specifically assessed (1) the 10-year survivorship in the whole cohort and in male and female patients, (2) serum concentrations of metal ions in patients with hip resurfacing who had not undergone revision surgery, and (3) potential influencing factors on the serum ion concentration.

METHODS

We retrospectively reviewed our first 95 patients who had 100 hip resurfacings performed from 1998 to 2001. The median age of the patients at surgery was 52 years (range, 28-69 years); 49% were men. We assessed the survival rate (revision for any reason as the end point), radiographic changes, and serum ion concentrations for cobalt, chromium, and molybdenum. The correlations between serum ion concentration and patient-related factors (age, sex, BMI, activity) and implant-related factors (implant size, cup inclination, stem-shaft angle) were investigated. The minimum followup was 9.3 years (mean, 10 years; range, 9.3-10.5 years).

RESULTS

The 10-year survivorship was 88% for the total cohort. The overall survival rate was greater in men (93%) than in women (84%). Median serum ion levels were 1.9 μg/L for chromium, 1.3 μg/L for cobalt, and 1.6 μg/L for molybdenum. Radiolucent lines around acetabular implants were observed in 4% and femoral neck thinning in 5%.

CONCLUSIONS

Although our overall failure rate was greater than anticipated, the relatively low serum ion levels and no revisions for pseudotumors in young male patients up to 10 years postoperatively provide some evidence of the suitability of hip resurfacing in this subgroup.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See the Instructions for Authors for a complete description of levels of evidence.

The accuracy and reliability of preoperative templating for metal-on-metal hip resurfacing

The purposes of this study were to evaluate the accuracy and reliability of preoperative templating on conventional radiographs (CRs) for metal-on-metal hip resurfacing and to determine the factors affecting the accuracy. Four observers templated 80 CRs on 2 separate occasions in a blinded fashion. Eight independent variables were evaluated to investigate their effect on the accuracy of templating. The overall accuracy of templating within one size of the actual component was 80.6% for the femoral component and 98.5% for the acetabular component. Overall, the intraobserver and interobserver reliability was fair to substantial (κ = 0.22-0.61). Using multiple regression analysis, surgical time was the only factor that affected the accuracy of predicting the size of the acetabular component (P = .019). We conclude that CR templating for metal-on-metal hip resurfacing is a useful method for preoperative planning of the sizes of the respective implants and that surgeon experience does play a significant role in the accuracy of predicting component size during templating.

Avoiding short-term femoral neck fracture with imageless computer navigation for hip resurfacing

BACKGROUND

Femoral neck fracture in hip resurfacing has been attributed to technical error during femoral head preparation. In the absence of fracture, several radiographic findings have been speculated to increase the risk of femoral component failure.

QUESTIONS/PURPOSES

We examined whether (1) the use of navigation to reduce technical errors during femoral head preparation reduces the incidence of femoral neck fractures in the short-term followup period; and (2) alignment of the femoral component with the use of computer navigation reduces the incidence of femoral neck thinning, femoral stem radiolucencies, and stem migration.

METHODS

We retrospectively reviewed the first 100 Birmingham Hip Resurfacings performed in 94 prospectively followed patients between October 2005 and November 2007. We examined all radiographs on last followup. Eighty-six patients of the 94 patients had a minimum followup of 2 years (mean, 2.5 years; range, 2-4.1 years).

RESULTS

There were no cases of femoral neck notching, varus femoral component alignment, or femoral neck fractures in the series. Neck thinning of greater than 10% was observed in three patients and perimetaphyseal stem lucencies were noted in 10 patients. In three patients, the metaphyseal stem showed varus migration relative to the postoperative stem-shaft angle at latest followup. There was one revision to a total hip arthroplasty for deep sepsis. The overall survivorship at 4 years was 99%.

CONCLUSIONS

The use of imageless computer navigation to reduce technical errors in hip resurfacing may reduce the incidence of femoral neck fracture in the short-term. However, neck thinning, stem radiolucencies, and stem migration remain radiographic sequelae of hip resurfacing despite the use of navigation for placement of the femoral component.

The effect of the cam deformity on the insertion of the femoral component in hip resurfacing

Surface arthroplasty simulations were generated using 3-dimensional computed tomographic scans from 61 consecutive patients presenting with idiopathic osteoarthritis to evaluate the change in femoral component positioning that would allow optimal alignment when resurfacing a cam-type deformity. Anatomical parameters were measured to quantify the influence of the deformity on the insertion technique of the femoral implant. A modified femoral head ratio was initially calculated from plain radiographs to define the severity of cam deformity in these patients. A severe deformity required more superior translation of the entry point and greater reaming depth to allow safe insertion with optimal implant alignment. This could be achieved while preserving the leg length, minimizing the component size, and maximizing the amount of host bone contact, although the horizontal femoral offset was reduced. These findings suggest that the femoral component can be safely inserted by modifying the surgical technique despite progressive deformity of the femoral head.

A comparison of conventional guidewire alignment jigs with imageless computer navigation in hip resurfacing arthroplasty

BACKGROUND

Correct positioning of the initial femoral guidewire is vital in order to prepare the femoral head properly for hip resurfacing. The purpose of the present investigation was to determine the accuracy and precision of the placement of the initial femoral guidewire with use of conventional alignment jigs and to compare the results with those of imageless computer navigation.

METHODS

Five commercially available jigs (two lateral pin jigs, two neck centering jigs, and one head planing jig) were obtained. Four surgeons used each jig and navigation three times to insert a guidewire in 10 degrees of relative valgus and neutral version into individual synthetic femora. A single surgeon then used each jig three times to align the initial guidewire in 10 degrees of relative valgus and neutral version in each of ten human cadaver femora. Radiographs of the synthetic and human femora were made to assess and compare guidewire inclination and version between conventional instrumentation and navigation.

RESULTS

Navigation provided ranges of error in the coronal guidewire alignment of up to eight times less than the conventional jigs, but both methods provided similar ranges of error for version. In both arms of the study, there were significant differences in coronal alignment accuracy between the two neck centering jigs. Next to navigation, one lateral pin jig provided the most accurate coronal placement of the initial guidewire whereas one neck centering jig provided the most precise coronal placement of the guidewire. Navigation was similar to conventional jigs in terms of the accuracy and precision of guidewire version.

CONCLUSIONS

In hip resurfacing arthroplasty, the choice of a femoral alignment device may influence the accuracy and precision of guidewire insertion, ultimately impacting femoral component placement. Imageless computer navigation can facilitate accurate and precise coronal alignment of the initial femoral guidewire, superior to that of conventional instrumentation.

CLINICAL RELEVANCE

The results of this study may aid surgeons in the selection of alignment instruments for placement of the initial femoral guidewire during hip resurfacing.

Biomechanical comparison of 3 possible fixation strategies to resist femoral neck shortening after fracture

In light of recent reports that patients with femoral neck shortening following fracture fixation are dissatisfied with their outcomes, the objective of this study was to compare the compressive strength, or resistance to shortening, of 3 possible strategies for stabilization of the femoral neck that should resist shortening. The proximal portion of 21 synthetic composite femurs were prepared to isolate the femoral neck for study. A 4-mm segment of the femoral neck was removed to simulate a transcervical comminuted fracture that would be expected to shorten under standard treatment conditions. These simulated fractures were fixed by 1 of 3 methods: a 3-screw configuration using parallel partially threaded screws augmented with an injectable hydroxyapatite bone substitute in the fracture site; a 3-screw configuration using parallel fully threaded screws; or a nonparallel 3-screw configuration using partially threaded screws. The specimens were tested in compression along the axis of the femoral neck, and the mean stiffness and load to failure values were calculated.The hydroxyapatite bone substitute-augmented partially threaded screw fixation construct resulted in the highest stiffness (1928+/-135 N/mm) and load to failure (6529+/-674 N), followed by the fully threaded screw construct (1210+/-166 N/mm and 3987+/-419 N, respectively), and finally the nonparallel construct (518+/-176 N/mm and 592+/-295 N, respectively) (P<.001 for all groups). This study supports further evaluation of hydroxyapatite bone substitute augmentation at the fracture site to prevent femoral neck shortening in femoral neck fractures receiving internal fixation.

Imageless computer navigation without pre-operative templating may lead to malpreparation of the femoral head in hip resurfacing

The computed neck-shaft angle and the size of the femoral component were recorded in 100 consecutive hip resurfacings using imageless computer-navigation and compared with the angle measured before operation and with actual component implanted. The reliability of the registration was further analysed using ten cadaver femora. The mean absolute difference between the measured and navigated neck-shaft angle was 16.3° (0° to 52°). Navigation underestimated the measured neck-shaft angle in 38 patients and the correct implant size in 11. Registration of the cadaver femora tended to overestimate the correct implant size and provided a low level of repeatability in computing the neck-shaft angle.

Prudent pre-operative planning is advisable for use in conjunction with imageless navigation since misleading information may be registered intraoperatively, which could lead to inappropriate sizing and positioning of the femoral component in hip resurfacing.

Imageless computer navigation for placement of the femoral component in resurfacing arthroplasty of the hip

We have investigated the accuracy of placement of the femoral component using imageless navigation in 100 consecutive Birmingham Hip Resurfacings. Pre-operative templating determined the native neck-shaft angle and planned stem-shaft angle of the implant. The latter were verified post-operatively using digital anteroposterior unilateral radiographs of the hip. The mean neck-shaft angle determined before operation was 132.7 degrees (118 degrees to 160 degrees ). The mean planned stem-shaft angle was a relative valgus alignment of 9.7 degrees (SD 2.6). The stem-shaft angle after operation differed from that planned by a mean of 2.8 degrees (SD 2.0) and in 86% of cases the final angle measured within +/- 5 degrees of that planned. We had no instances of notching of the neck or varus alignment of the implant in our series. A learning curve was observed in the time taken for navigation, but not for accurate placement of the implant. Navigation in hip resurfacing may afford the surgeon a reliable and accurate method of placement of the femoral component.